This invention relates to calcium carbonate minerals and, more particularly, is concerned with finely comminuted calcium carbonate minerals which can be used, for example, as pigments in paper coating or emulsion paints or as fillers in rubber and plastics materials. The term "calcium carbonate mineral" as used herein includes natural chalk, vein calcite, calcite marble, limestone and dolomite.
It is known to grind calcium carbonate minerals to a fine particle size, for example to a size such that at least 60% by weight of the calcium carbonate mineral consists of particles having an equivalent spherical diameter smaller than 2 .mu.m, by forming a concentrated aqueous suspension of the calcium carbonate mineral in the presence of a deflocculating agent, and subjecting the deflocculated aqueous suspension to attrition grinding with a particulate grinding medium, such as a silica sand.
A convenient form in which a finely ground calcium carbonate mineral can be transported and sold is as a deflocculated aqueous suspension having a solids content such that little, if any, sedimentation of the particles occurs during transit and storage. In order to meet this requirement the solids content of the suspension should be high, i.e. about 65% or more by weight, and preferably in the range of from 70% to 85% by weight. Also, it is generally found that the suspension should not contain more than about 1% by weight of particles larger than 50 .mu.m and that at least 5 horsepower hours of energy per ton of dry calcium carbonate mineral should be dissipated in the suspension to ensure that a uniform and stable suspension is formed. It would be advantageous to be able to perform all the necessary operations on the calcium mineral in an aqueous suspension having a solids content close to that required for the slurry which is to be transported and sold, because if any operation has to be performed on a more dilute suspension it is necessary to add water to the raw calcium carbonate mineral (which may contain in the natural state up to about 20% by weight of water), and then dewater the suspension by some means. Such dewatering operations are difficult and expensive.
A problem which arises in the production of aqueous suspensions of finely ground calcium carbonate minerals at high solids contents is that even after grinding with a particulate grinding medium and passing the suspension of ground calcium carbonate mineral through a sieve to remove substantially all particles larger than 50 .mu.m, the calcium carbonate still contains a small proportion, say up to about 5% by weight, of particles smaller than 50 .mu.m but larger than 10 .mu.m. Among the particles in this size range there are inevitably particles of hard impurities, such as silica, which cause abrasion problems when the ground calcium carbonate is used as a pigment or filler.